"I love you" is more geared towards people you admire or like a lot, such as family or friends. It's a term of endearment used a lot of times, but you can love people and still not be in love with them, per se.
However, saying "I'm in love with you" shows that you feel a great sense of adoration romantically to the person that you say it to. This is love on a much deeper level and it's more intimate.
However, you can use both terms to the person that you are in love with, but you shouldn't saying "I'm in love with you" to someone who is a family member or someone that you consider to be just a friend. 
